This invention relates to semiconductor devices used in high power switching applications. More specifically, it relates to bipolar power transistors such as those used in transcalent devices.
Bipolar transistors operated at high power densities are subject to a failure mode termed second breakdown. This breakdown is characterized by a sudden decrease in emitter-collector voltage and a simultaneous increase in emitter current, such that unless the power is rapidly removed, the device will be destroyed or significantly degraded by overheating. Second breakdown is caused by current concentrations in local areas of the emitter/base junction creating local overheating. Once a localized hot spot occurs, positive thermal feedback begins; the hot region injects more, thereby getting hotter, thereby injecting more. If the available power is limited, the peak temperature remains below a critical value. However, if the peak temperature reaches a value such that local base-collector leakage current reaches base current magnitude, the device regenerates into second breakdown.
Second breakdown can occur during either forward or reverse bias and typically initiates at a central portion of the emitter/base junction area. Various forms of series resistance emitter ballasting and base ballasting have been suggested in an effort to reduce hot spot formation, although they have been only partially effective in protecting against reverse bias second breakdown. Shorting the base to the emitter through a resistance is another known technique for generally improving second breakdown characteristics of power transistors. However, this technique, known as shunt ballasting, is not wholly satisfactory because the base region typically has a relatively low resistivity, thereby permitting an excessive base shunt current which degrades the transistor current gain (hFE).
The present invention discloses a shunt ballasting structure which provides forward and reverse second breakdown protection with no significant degradation in current gain at normal power operating levels.